Method and system for dimensional uniformity using charged particle beam lithography

ABSTRACT

A method for mask process correction or forming a pattern on a reticle using charged particle beam lithography is disclosed, where the reticle is to be used in an optical lithographic process to form a pattern on a wafer, where sensitivity of the wafer pattern is calculated with respect to changes in dimension of the reticle pattern, and where pattern exposure information is modified to increase edge slope of the reticle pattern where sensitivity of the wafer pattern is high. A method for fracturing or mask data preparation is also disclosed, where pattern exposure information is determined that can form a pattern on a reticle using charged particle beam lithography, where the reticle is to be used in an optical lithographic process to form a pattern on a wafer, and where sensitivity of the wafer pattern is calculated with respect to changes in dimension of the reticle pattern.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/331,008 filed on Jul. 14, 2014 and entitled “Method and System ForDimensional Uniformity Using Charged Particle Beam Lithography”; whichis a continuation-in-part of U.S. patent application Ser. No. 13/801,571filed on Mar. 13, 2013 and entitled “Method and System For DimensionalUniformity Using Charged Particle Beam Lithography” and published asU.S. Patent Application Pub. 2014-0129997; which are hereby incorporatedby reference for all purposes. U.S. patent application Ser. No.13/801,571 claims priority from U.S. Provisional Patent Application Ser.No. 61/724,232 filed on Nov. 8, 2012 and entitled “Method and System ForImproving Critical Dimension Uniformity Using Shaped Beam Lithography”;and is related to U.S. patent application Ser. No. 13/801,554 filed Mar.13, 2013, entitled “Method and System For Dimensional Uniformity UsingCharged Particle Beam Lithography” and published as U.S. PatentApplication Pub. 2014-0129996; both of which are hereby incorporated byreference for all purposes. U.S. patent application Ser. No. 14/331,008is also a continuation-in-part of U.S. patent application Ser. No.13/862,471 filed on Apr. 15, 2013 and entitled “Method and System forForming Patterns Using Charged Particle Beam Lithography” and publishedas U.S. Patent Application Pub. 2013-0283217; which claims priority fromU.S. Provisional Patent Application No. 61/625,789 filed on Apr. 18,2012, entitled “Method And System For Forming Patterns Using ChargedParticle Beam Lithography,” both of which are hereby incorporated byreference for all purposes.

BACKGROUND OF THE DISCLOSURE

In the production or manufacturing of semiconductor devices, such asintegrated circuits, optical lithography may be used to fabricate thesemiconductor devices. Optical lithography is a printing process inwhich a lithographic mask or photomask manufactured from a reticle isused to transfer patterns to a substrate such as a semiconductor orsilicon wafer to create the integrated circuit (I.C.). Other substratescould include flat panel displays, holographic masks or even otherreticles. While conventional optical lithography uses a light sourcehaving a wavelength of 193 nm, extreme ultraviolet (EUV) or X-raylithography are also considered types of optical lithography in thisapplication. The reticle or multiple reticles may contain a circuitpattern corresponding to an individual layer of the integrated circuit,and this pattern can be imaged onto a certain area on the substrate thathas been coated with a layer of radiation-sensitive material known asphotoresist or resist. Once the patterned layer is transferred the layermay undergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Eventually, a combination of multiplesof devices or integrated circuits will be present on the substrate.These integrated circuits may then be separated from one another bydicing or sawing and then may be mounted into individual packages. Inthe more general case, the patterns on the substrate may be used todefine artifacts such as display pixels, holograms, directedself-assembly (DSA) guard bands, or magnetic recording heads.Conventional optical lithography writing machines typically reduce thephotomask pattern by a factor of four during the optical lithographicprocess. Therefore, patterns formed on the reticle or mask must be fourtimes larger than the size of the desired pattern on the substrate orwafer.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, non-optical methods may be used to transfer apattern on a lithographic mask to a substrate such as a silicon wafer.Nanoimprint lithography (NIL) is an example of a non-optical lithographyprocess. In nanoimprint lithography, a lithographic mask pattern istransferred to a surface through contact of the lithography mask withthe surface.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, maskless direct write may also be used to fabricatethe semiconductor devices. Maskless direct write is a printing processin which charged particle beam lithography is used to transfer patternsto a substrate such as a semiconductor or silicon wafer to create theintegrated circuit. Other substrates could include flat panel displays,imprint masks for nano-imprinting, or even reticles. Desired patterns ofa layer are written directly on the surface, which in this case is alsothe substrate. Once the patterned layer is transferred the layer mayundergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Some of the layers may be written usingoptical lithography while others may be written using maskless directwrite to fabricate the same substrate. Eventually, a combination ofmultiples of devices or integrated circuits will be present on thesubstrate. These integrated circuits are then separated from one anotherby dicing or sawing and then mounted into individual packages. In themore general case, the patterns on the surface may be used to defineartifacts such as display pixels, holograms or magnetic recording heads.

Two common types of charged particle beam lithography are variableshaped beam (VSB) and character projection (CP). These are bothsub-categories of shaped beam charged particle beam lithography, inwhich a precise electron beam is shaped and steered so as to expose aresist-coated surface, such as the surface of a wafer or the surface ofa reticle. In VSB, these shapes are simple shapes, usually limited torectangles of certain minimum and maximum sizes and with sides which areparallel to the axes of a Cartesian coordinate plane (i.e. of“manhattan” orientation), and 45 degree right triangles (i.e. triangleswith their three internal angles being 45 degrees, 45 degrees, and 90degrees) of certain minimum and maximum sizes. At predeterminedlocations, doses of electrons are shot into the resist with these simpleshapes. The total writing time for this type of system increases withthe number of shots. In character projection (CP), there is a stencil inthe system that has in it a variety of apertures or characters which maybe complex shapes such as rectilinear, arbitrary-angled linear,circular, nearly circular, annular, nearly annular, oval, nearly oval,partially circular, partially nearly circular, partially annular,partially nearly annular, partially nearly oval, or arbitrarycurvilinear shapes, and which may be a connected set of complex shapesor a group of disjointed sets of a connected set of complex shapes. Anelectron beam can be shot through a character on the stencil toefficiently produce more complex patterns on the reticle. In theory,such a system can be faster than a VSB system because it can shoot morecomplex shapes with each time-consuming shot. Thus, an E-shaped patternshot with a VSB system takes four shots, but the same E-shaped patterncan be shot with one shot with a character projection system. Note thatVSB systems can be thought of as a special (simple) case of characterprojection, where the characters are just simple characters, usuallyrectangles or 45-45-90 degree triangles. It is also possible topartially expose a character. This can be done by, for instance,blocking part of the particle beam. For example, the E-shaped patterndescribed above can be partially exposed as an F-shaped pattern or anI-shaped pattern, where different parts of the beam are cut off by anaperture. This is the same mechanism as how various sized rectangles canbe shot using VSB. In this disclosure, partial projection is used tomean both character projection and VSB projection. Shaped beam chargedparticle beam lithography may use either a single shaped beam, or mayuse a plurality of shaped beams simultaneously exposing the surface, theplurality of shaped beams producing a higher writing speed than a singleshaped beam.

As indicated, in lithography the lithographic mask or reticle comprisesgeometric patterns corresponding to the circuit components to beintegrated onto a substrate. The patterns used to manufacture thereticle may be generated utilizing computer-aided design (CAD) softwareor programs. In designing the patterns the CAD program may follow a setof pre-determined design rules in order to create the reticle. Theserules are set by processing, design, and end-use limitations. An exampleof an end-use limitation is defining the geometry of a transistor in away in which it cannot sufficiently operate at the required supplyvoltage. In particular, design rules can define the space tolerancebetween circuit devices or interconnect lines. The design rules are, forexample, used to ensure that the circuit devices or lines do notinteract with one another in an undesirable manner. For example, thedesign rules are used so that lines do not get too close to each otherin a way that may cause a short circuit. The design rule limitationsreflect, among other things, the smallest dimensions that can bereliably fabricated. When referring to these small dimensions, oneusually introduces the concept of a critical dimension. These are, forinstance, defined as the smallest width of a line or the smallest spacebetween two lines, those dimensions requiring exquisite control.

One goal in integrated circuit fabrication by optical lithography is toreproduce the original circuit design on the substrate by use of thereticle. Integrated circuit fabricators are always attempting to use thesemiconductor wafer real estate as efficiently as possible. Engineerskeep shrinking the size of the circuits to allow the integrated circuitsto contain more circuit elements and to use less power. As the size ofan integrated circuit critical dimension is reduced and its circuitdensity increases, the critical dimension of the circuit pattern orphysical design approaches the resolution limit of the optical exposuretool used in conventional optical lithography. As the criticaldimensions of the circuit pattern become smaller and approach theresolution value of the exposure tool, the accurate transcription of thephysical design to the actual circuit pattern developed on the resistlayer becomes difficult. To further the use of optical lithography totransfer patterns having features that are smaller than the lightwavelength used in the optical lithography process, a process known asoptical proximity correction (OPC) has been developed. OPC alters thephysical design to compensate for distortions caused by effects such asoptical diffraction and the optical interaction of features withproximate features. OPC includes all resolution enhancement technologiesperformed with a reticle.

Inverse lithography technology (ILT) is one type of OPC technique. ILTis a process in which a pattern to be formed on a reticle is directlycomputed from a pattern which is desired to be formed on a substratesuch as a silicon wafer. This may include simulating the opticallithography process in the reverse direction, using the desired patternon the substrate as input. ILT-computed reticle patterns may be purelycurvilinear—i.e. completely non-rectilinear—and may include circular,nearly circular, annular, nearly annular, oval and/or nearly ovalpatterns. Since these ideal ILT curvilinear patterns are difficult andexpensive to form on a reticle using conventional techniques,rectilinear approximations or rectilinearizations of the idealcurvilinear patterns may be used. The rectilinear approximationsdecrease accuracy, however, compared to the ideal ILT curvilinearpatterns. Additionally, if the rectilinear approximations are producedfrom the ideal ILT curvilinear patterns, the overall calculation time isincreased compared to ideal ILT curvilinear patterns. In this disclosureILT, OPC, source mask optimization (SMO), and computational lithographyare terms that are used interchangeably.

There are a number of technologies used for forming patterns on areticle, including using optical lithography or charged particle beamlithography. The most commonly used system is the variable shaped beam(VSB), where, as described above, doses of electrons with simple shapessuch as manhattan rectangles and 45-degree right triangles expose aresist-coated reticle surface. In conventional mask writing, the dosesor shots of electrons are designed to avoid overlap wherever possible,so as to greatly simplify calculation of how the resist on the reticlewill register the pattern. Similarly, the set of shots is designed so asto completely cover the pattern area that is to be formed on thereticle. U.S. Pat. No. 7,754,401, owned by the assignee of the presentpatent application and incorporated by reference for all purposes,discloses a method of mask writing in which intentional shot overlap forwriting patterns is used. When overlapping shots are used, chargedparticle beam simulation can be used to determine the pattern that theresist on the reticle will register. Use of overlapping shots may allowpatterns to be written with reduced shot count. U.S. Pat. No. 7,754,401also discloses use of dose modulation, where the assigned dosages ofshots vary with respect to the dosages of other shots. The termmodel-based fracturing is used to describe the process of determiningshots using the techniques of U.S. Pat. No. 7,754,401.

SUMMARY OF THE DISCLOSURE

A method for mask process correction or forming a pattern on aresist-coated reticle using charged particle beam lithography isdisclosed, where the reticle is to be used in an optical lithographicprocess to form a pattern on a wafer, where the sensitivity of the waferpattern is calculated with respect to changes in dimension of thereticle pattern and where the pattern exposure information is modifiedto increase edge slope of the reticle pattern where sensitivity of thewafer pattern is high.

A method for fracturing or mask data preparation is also disclosed,where pattern exposure information is determined that can form a patternon a resist-coated reticle using charged particle beam lithography,where the reticle is to be used in an optical lithographic process toform a pattern on a wafer, and where the sensitivity of the waferpattern is calculated with respect to changes in dimension of thereticle pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a variable shaped beam (VSB) chargedparticle beam system;

FIG. 2 illustrates examples of patterns formed using various sized VSBshots, and the cross sectional dosage for each pattern;

FIG. 3 illustrates a graph of mask critical dimension (CD) error as afunction of dosage variation, for mask features of different sizes;

FIG. 4 illustrates an exemplary conceptual flow diagram for calculatingthe sensitivity of a wafer CD with respect the dosage variation ofcharged particle beam shots used to write the mask pattern used to formthe wafer pattern;

FIG. 5 illustrates a conceptual flow diagram of a method for mask datapreparation in one embodiment;

FIG. 6 illustrates a conceptual flow diagram of a method for maskprocess correction in one embodiment;

FIG. 7A illustrates an example of a set of conventional shots that maybe used to form a circular pattern such as a sub-100 nm pattern;

FIG. 7B illustrates an example of a set of overlapping shots that may beused to form a circular pattern according to an embodiment of thecurrent disclosure;

FIG. 8 illustrates an exemplary computing hardware device used inembodiments of the methods; and

FIG. 9 illustrates an exemplary conceptual flow diagram for calculatingthe sensitivity of a wafer pattern dimension with respect to changes ina mask pattern dimension.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure is related to lithography, and more particularlyto the design and manufacture of a surface which may be a reticle, awafer, or any other surface, using charged particle beam lithography.

Referring now to the drawings, wherein like numbers refer to like items,FIG. 1 illustrates an embodiment of a lithography system, such as acharged particle beam writer system, in this case an electron beamwriter system 10, that employs a variable shaped beam (VSB) tomanufacture a surface 12. The electron beam writer system 10 has anelectron beam source 14 that projects an electron beam 16 toward anaperture plate 18. The plate 18 has an aperture 20 formed therein whichallows the electron beam 16 to pass. Once the electron beam 16 passesthrough the aperture 20 it is directed or deflected by a system oflenses (not shown) as electron beam 22 toward another rectangularaperture plate or stencil mask 24. The stencil 24 has formed therein anumber of openings or apertures 26 that define various simple shapessuch as rectangles and triangles. Each aperture 26 formed in the stencil24 may be used to form a pattern in the surface 12 of a substrate 34,such as a silicon wafer, a reticle or other substrate. An electron beam30 emerges from one of the apertures 26 and passes through anelectromagnetic or electrostatic reduction lens 38, which reduces thesize of the pattern emerging from the aperture 26. In commonly availablecharged particle beam writer systems, the reduction factor is between 10and 60. The reduced electron beam 40 emerges from the reduction lens 38and is directed by a series of deflectors 42 onto the surface 12 as apattern 28. The surface 12 is coated with resist (not shown) whichreacts with the electron beam 40. The electron beam 22 may be directedto overlap a variable portion of an aperture 26, affecting the size andshape of the pattern 28. Blanking plates (not shown) are used to deflectthe beam 16 or the shaped beam 22 so to prevent the electron beam fromreaching the surface 12 during a period after each shot when the lensesdirecting the beam 22 and the deflectors 42 are being re-adjusted forthe succeeding shot. Typically the blanking plates are positioned so asto deflect the electron beam 16 to prevent it from illuminating aperture20. The blanking period may be a fixed length of time, or it may varydepending, for example, on how much the deflector 42 must be re-adjustedfor the position of the succeeding shot.

In electron beam writer system 10, the substrate 34 is mounted on amovable platform 32. The platform 32 allows substrate 34 to berepositioned so that patterns which are larger than the maximumdeflection capability or field size of the charged particle beam 40 maybe written to surface 12 in a series of subfields, where each subfieldis within the capability of deflector 42 to deflect the beam 40. In oneembodiment the substrate 34 may be a reticle. In this embodiment, thereticle, after being exposed with the pattern, undergoes variousmanufacturing steps through which it becomes a lithographic mask orphotomask. The mask may then be used in an optical lithography machineto project an image of the reticle pattern 28, generally reduced insize, onto a silicon wafer to produce an integrated circuit. Moregenerally, the mask is used in another device or machine to transfer thepattern 28 on to a substrate (not illustrated).

Other types of charged particle beam writers include characterprojection and multi-beam. In a multi-beam writer the pattern is createdby a plurality of charged particle beams which deposit dosage on aresist-coated surface. The surface may remain stationary or maycontinuously slowly move. Many thousands of beams may be used so as toachieve a high writing speed.

The minimum size pattern that can be projected with reasonable accuracyonto a surface 12 is limited by a variety of short-range physicaleffects associated with the electron beam writer system 10 and with thesurface 12, which normally comprises a resist coating on the substrate34. These effects include forward scattering, Coulomb effect, and resistdiffusion. Beam blur, also called β_(f), is a term used to include allof these short-range effects. The most modern electron beam writersystems can achieve an effective beam blur radius or β_(f) in the rangeof 20 nm to 30 nm. Forward scattering may constitute one quarter to onehalf of the total beam blur. Modern electron beam writer systems containnumerous mechanisms to reduce each of the constituent pieces of beamblur to a minimum. Since some components of beam blur are a function ofthe calibration level of a particle beam writer, the β_(f) of twoparticle beam writers of the same design may differ. The diffusioncharacteristics of resists may also vary. Variation of β_(f) based onshot size or shot dose can be simulated and systemically accounted for.But there are other effects that cannot or are not accounted for, andthey appear as random variation.

The shot dosage of a shaped beam charged particle beam writer such as anelectron beam writer system is a function of the intensity of the beamsource 14 and the exposure time for each shot. Typically the beamintensity remains nominally fixed, and the exposure time is varied toobtain variable shot dosages. The exposure time may be varied tocompensate for various long-range effects such as backscatter, fogging,and loading effects in a process called proximity effect correction(PEC). Electron beam writer systems usually allow setting an overalldosage, called a base dosage, which affects all shots in an exposurepass. Some electron beam writer systems perform dosage compensationcalculations within the electron beam writer system itself, and do notallow the dosage of each shot to be assigned individually as part of theinput shot list, the input shots therefore having unassigned shotdosages. In such electron beam writer systems all shots implicitly havethe base dosage, before PEC. Other electron beam writer systems do allowexplicit dosage assignment on a shot-by-shot basis. In electron beamwriter systems that allow shot-by-shot dosage assignment, the number ofavailable dosage levels may be 64 to 4096 or more, or there may be arelatively few available dosage levels, such as 3 to 8 levels.

Conventionally, shots are designed so as to completely cover an inputpattern with rectangular shots, while avoiding shot overlap whereverpossible within an exposure pass. Also, all shots are designed to have anormal dosage, which is a dosage at which a relatively large rectangularshot, in the absence of long-range effects, will produce a pattern onthe surface which is the same size as is the shot size. Some electronbeam writer systems enforce this methodology by not allowing shots tooverlap within an exposure pass.

In exposing, for example, a repeated pattern on a surface using chargedparticle beam lithography, the size of each pattern instance, asmeasured on the final manufactured surface, will be slightly different,due to manufacturing variations. The amount of the size variation is anessential manufacturing optimization criterion. In current mask masking,a root mean square (RMS) variation of no more than 1 nm (1 sigma) inpattern size may be desired. More size variation translates to morevariation in circuit performance, leading to higher design margins beingrequired, making it increasingly difficult to design faster, lower-powerintegrated circuits. This variation is referred to as critical dimension(CD) variation. A low CD variation is desirable, and indicates thatmanufacturing variations will produce relatively small size variationson the final manufactured surface. In the smaller scale, the effects ofa high CD variation may be observed as line edge roughness (LER). LER iscaused by each part of a line edge being slightly differentlymanufactured, leading to some waviness in a line that is intended tohave a straight edge. CD variation is, among other things, inverselyrelated to the slope of the dosage curve at the resist threshold, whichis called edge slope. Therefore, edge slope, or dose margin, is acritical optimization factor for particle beam writing of surfaces. Inthis disclosure, edge slope and dose margin are terms that are usedinterchangeably.

As described above, process variations can cause the width of a patternon a photomask to vary from the intended or target width. The patternwidth variation on the photomask will cause a pattern width variation ona wafer which has been exposed with the photomask using an opticallithographic process. The sensitivity of the wafer pattern width tovariations in photomask pattern width is called mask edge error factor,or MEEF. In an optical lithography system using a 4× photomask, wherethe optical lithographic process projects a 4× reduced version of thephotomask pattern onto the wafer, a MEEF of 1, for example means thatfor each 1 nm error in pattern width on a photomask, the pattern widthon the wafer will change by 0.25 nm. A MEEF of 2 means that for a 1 nmerror in photomask pattern width, the pattern width on the wafer willchange by 0.5 nm. For the smallest integrated circuits processes, MEEFmay be greater than 2, and for ideal ILT patterns MEEF may be 3.0 to 3.5or higher. This relationship can be expressed in equation form as

$\begin{matrix}{{\Delta \; {CDwafer}} = {\frac{MEEF}{R}\Delta \; {CDmask}}} & (1)\end{matrix}$

where R is the reduction factor, typically 4 for integrated circuitfabrication. The usefulness of MEEF has rested on two assumptions:

-   -   That different mask shapes have similar sensitivity to errors    -   Mask error can be approximated by a uniform bias

FIG. 2 illustrates examples of four example mask patterns formed bysquare VSB shots of different sizes, and a graph of the longitudinaldosage profile through the centerline of each pattern. Pattern 208 is asquare 200 nm shot, pattern 206 is a square 100 nm shot, pattern 204 isa square 80 nm shot, and pattern 202 is a square 60 nm shot. Beam blurcauses the corners of all shots to be rounded, sufficiently so thatpatterns formed with the smaller shots register on the resist ascircles. The dosage profile graph 210 illustrates the longitudinaldosage profile through line 200. The vertical axis of dosage profilegraph 210 is the fraction of normal dosage. Dosage profile graph 210illustrates a resist threshold 212 of 0.5 of normal dosage. Thecalculated edge slope of pattern 208 at x-coordinates “g” and “h” is1.89% of normal dosage per nm. The calculated edge slope of pattern 206at x-coordinates “e” and “f” is 1.85% of normal dosage per nm. Thecalculated edge slope of pattern 204 at x-coordinates “c” and “d” is1.75% of normal dosage per nm. The calculated edge slope of pattern 202at x-coordinates “a” and “b” is 1.49% of normal dosage per nm. Thesmaller edge slope for the smaller patterns 202 and 204 will cause alarger CD change in these patterns for a given change in dosage,compared to the larger patterns 206 and 208.

Therefore, the above equation (1) is no longer helpful in predicting thesensitivity of the wafer pattern to a change in dosage for patternssmaller than about 100 nm. In equation form, the relationship between achange in dosage and the resulting mask CD change can be expressed as

ΔCDmask=DoseEdgeSbpe·ΔDose   (2)

Using equation (2) with charged particle beam simulation, with the onlysimulated effect being a forward scattering radius of 30 nm, therelationship between mask feature size and mask ΔCD can be derived. Thisis illustrated in FIG. 3. As can be seen, as the feature size of a maskpattern falls below 100 nm, the mask ΔCD goes up rapidly for a givendosage variation.

More generally, ΔDose may be caused either by a change in actual chargedparticle dosage received by the resist, or by a change in the dosagethreshold at which the resist will register a pattern. In thisdisclosure, the terms “dosage change” and “resist exposure” both referto both of these phenomena. An increase in resist exposure may beproduced either by an increase in actual charged particle dosage or by alowering of the resist threshold. Similarly, a decrease in resistexposure may be produced either by a decrease in actual charged particledosage or by an increase in the resist threshold.

Given that the conventional MEEF method will not accurately predictwafer CD sensitivity for a change in dose for shapes <100 nm, there is aneed for alternate methods to determine wafer CD sensitivity to changesin resist exposure. In the current disclosure, the wafer CD sensitivityto resist exposure change is calculated in what can be viewed as atwo-step process:

-   -   1. calculate the sensitivity of the mask CD to a change in        resist exposure    -   2. calculate the sensitivity of the wafer CD to a change in mask        CD        Step 1 may be accomplished using, for example, charged particle        beam simulation to calculate a mask pattern for each of two        dosages: a minimum resist exposure and a maximum resist        exposure. Step 2 may be accomplished using, for example,        lithography simulation, starting from the two mask patterns        calculated in step 1. Alternatively, reticle patterns may be        physically exposed at a lower-limit resist exposure and an        upper-limit resist exposure, and the resulting mask used to        print patterns on a substrate using optical lithography, after        which the CDs of the substrate patterns can be measured. The        details of the two steps will be described below.

The calculation of step 1, called charged particle beam simulationabove, and more commonly called E-beam simulation, may be moreaccurately described as a mask process simulation step. Charged particlebeam simulation must take into account effects associated with thecharged particle beam exposure process itself, such as forwardscattering, backward scattering, resist diffusion, resist charging,Coulomb effect and fogging, as well as non-exposure effects such asdevelop, bake and etch efforts, including, for example, loading.Similarly, step 2 may be described as a wafer process simulation step,although it is more commonly called lithography simulation or lithosimulation.

The conceptual flow diagram of FIG. 4 illustrates an exemplary method400 for calculating wafer CD sensitivity to variation in resistexposure, when using a shaped beam charged particle beam writer. Theprimary input to the process is a shot list 402. Additional inputs areresist exposure information “A” 430, resist exposure information “B”432, and wafer process information 416. Resist exposure information 430and 432 may comprise resist threshold information. In some embodiments,resist exposure information 430 and 432 may also comprise chargedparticle beam dosage information. In other embodiments, shot list 402may contain the charged particle beam dosage information. In yet otherembodiments, shot list 402 may contain charged particle beam dosageinformation, which is combined with base dosage information in resistexposure information 430 and 432 to determine an actual charged particlebeam dosage. In step 408 the shot list 402 is simulated using the resistexposure information “A” 430. Charged particle beam simulation is usedfor the simulation. Simulation step 408 creates simulated mask patterns410. In step 412 lithography simulation is performed on simulated maskpatterns 410 using wafer process information 416 to create simulatedwafer patterns 414. Similar steps are performed for the resist exposureinformation “B” 432. Charged particle beam simulation 418 is performedon shot list 402 using resist exposure information “B” 432. The chargedparticle beam simulation 418 creates simulated mask patterns 420. Instep 422 lithography simulation is performed on simulated mask patterns420, using wafer process information 416, to create simulated waferpatterns 424. Finally, in step 426 simulated wafer patterns 414 arecompared with simulated wafer patterns 424 to calculate the ΔCD 428 ofthe wafer patterns. The ΔCD 428 is the change in wafer CD with thechange in dosage from resist exposure information “A” 430 to resistexposure information “B” 432. CD comparison step 426 may compare thewafer pattern CDs at many places in the wafer patterns. In someembodiments the ΔCD 428 may be divided by the difference between resistexposure information 430 and resist exposure information 432, obtainingΔCD per unit change in resist exposure.

In other embodiments when using a multi-beam charged particle beamwriter, the input to method 400 will be pattern exposure information forthe multi-beam charged particle beam writer.

In another embodiment, the conceptual flow diagram of FIG. 9 illustratesan exemplary method 900 for calculating wafer dimensional sensitivity tochanges in mask pattern dimension, when using a shaped beam chargedparticle beam writer. The primary input to the process is a shot list902. Additional inputs are resist exposure information 904, waferprocess information 916, and mask bias value 908. Resist exposureinformation 904 may comprise resist threshold information. In someembodiments, resist exposure information 904 may also comprise chargedparticle beam dosage information. In other embodiments, shot list 902may contain the charged particle beam dosage information. In yet otherembodiments, shot list 902 may contain charged particle beam dosageinformation, which is combined with base dosage information in resistexposure information 904 to determine an actual charged particle beamdosage. In step 906 the shot list 902 is simulated using the resistexposure information 904. Charged particle beam simulation is used forthe simulation. Simulation step 906 creates simulated mask patterns 910.In step 912 lithography simulation is performed on simulated maskpatterns 910 using wafer process information 916 to create simulatedwafer patterns 914. In step 918 a pattern edge dimensional bias, whichis input as mask bias value 908, is applied to the simulated maskpatterns 910 to create biased mask patterns 920. In step 922 lithographysimulation is performed on the biased mask patterns 920 using waferprocess information 916 to create simulated wafer patterns 924. In step926 simulated wafer patterns 914 are compared with simulated waferpatterns 924 to calculate the ΔCD 928 of the wafer patterns. The ΔCD 928is the change in wafer dimension with the change in mask dimensioncaused by dimensional bias step 918. CD comparison step 926 may comparethe wafer pattern dimensions at many places in the wafer patterns. Asexplained above, the ΔCD may be significantly higher for wafer patternsexposed using mask patterns with feature sizes less than 100 nm, in maskdimensions. In step 930, the ΔCD 928 is divided by the mask bias valueto determine a calculated sensitivity 932. Calculated sensitivity 932 isthe change in wafer pattern dimension per unit change in mask dimension.

In those areas of the simulated wafer pattern where wafer sensitivitywith respect to mask writing dosage is found to be too high, asdetermined, for example, using method 400, a method is needed to reducethe wafer sensitivity in these problem areas. Since, in general, thelarger-than-desired wafer pattern ΔCD is the result of a relativelylarge mask pattern ΔCD, the mask pattern sensitivity to a change inresist exposure must be lowered. Equation (2) above indicates that thiscan be accomplished by increasing the edge slope at the perimeter of apattern or patterns in a problem area. One method of increasing edgeslope is to increase the dosage of the entire exposed pattern, such asby increasing the dosage of all shots if using shaped beam chargedparticle beam lithography. This method has the disadvantages ofincreasing write time, and also increasing backscattering, therebylowering the edge slope of all patterns. It is therefore best to adddosage only where necessary, such as around the perimeter of the problemshapes. However, FIG. 2 illustrates that using small or narrow shotsaround the perimeter of a shape will tend to increase the sensitivity ofthe shape's CD to a change in resist exposure. An alternative istherefore to generate overlapping shots to increase the dosage near theperimeter of the shape.

FIGS. 7A&B illustrate examples of two configurations of VSB shots thatmay be used to form a circular pattern, such as may be generated by ILTOPC. FIG. 7A illustrates an example of a set of conventionalnon-overlapping shots 702. Set of shots 702 consists of 7 shots. Pattern704 illustrates a pattern that set of shots 702 may form on a reticle ormask. In this example pattern 704 is less than 100 nm in diameter.Charged particle beam simulation shows that pattern 704 has areas of lowedge slope 706 along portions of its perimeter. In these areas theperimeter of pattern 704 is too sensitive to changes in resist exposure.FIG. 7B illustrates an example of another set of shots 712 determinedaccording to an embodiment of the current disclosure. Set of shots 712consists of 6 overlapping shots. By allowing shot overlap within anexposure pass, set of shots 712 does not use numerous narrow shots asdoes set of shots 702. By avoiding the narrow shots which are moresensitive to resist exposure changes, set of shots 712 forms circularpattern 714 which does not have areas of low edge slope.

In those embodiments where sensitivity of the wafer pattern iscalculated with respect to changes in the mask pattern, such as by usingmethod 900, when areas of high calculated wafer pattern sensitivity arefound that are, for example, higher than a predetermined threshold, thenthe pattern exposure information can be modified using, for example,techniques such as illustrated in FIGS. 7A & B and described above, toimprove the edge slope of the reticle or mask in the areas where thewafer pattern is highly sensitive to mask dimensional changes.Increasing the edge slope on the reticle or mask will reduce thedimensional variation on the mask. Expressed differently, the increasededge slope on the mask will improve CD uniformity on the mask, whichwill improve the wafer pattern CD uniformity.

FIG. 5 is a conceptual flow diagram 500 for forming patterns onsubstrates such as a silicon wafer using optical lithography, accordingto another embodiment of the current disclosure. In a first step 502, aphysical design, such as a physical design of an integrated circuit, iscreated. This can include determining the logic gates, transistors,metal layers, and other items that are required to be found in aphysical design such as the physical design of an integrated circuit.Next, in a step 504, optical proximity correction (OPC) is done on thepatterns in the physical design 502 or on a portion of the patterns inthe physical design to create a mask design 506. OPC alters the physicaldesign to compensate for distortions caused by effects such as opticaldiffraction and the optical interaction of features with proximatefeatures. In some embodiments, OPC step 504 may comprise ILT. In step508, the mask design 506 is fractured into a set of charged particlebeam shots for a shaped beam charged particle beam system to create shotlist 516. In some embodiments the shots will be VSB shots. In otherembodiments the shots will be CP shots or a combination of VSB and CPshots. Shot list 516 may comprise shots for a single exposure pass, orfor multiple exposure passes. MDP shot generation step 508 may comprisecalculating the sensitivity of dimensions for wafer patterns withrespect to variations in resist exposure, using the shots beinggenerated, the sensitivity calculations being illustrated as step 512,where the wafer patterns are to be formed in step 526 below. In oneembodiment the method illustrated in flow diagram 400 (FIG. 4) may beused for step 512. MDP step 508 may comprise using model-basedfracturing techniques. MDP step 508 may comprise generating a shotconfiguration which produces higher dosage at the perimeter of one ormore patterns than in the interior of the patterns. In some embodiments,shots will be generated so as to produce a wafer pattern sensitivity toresist exposure variation that is below a pre-determined limit. In otherembodiments, MDP step 508 may comprise calculating the sensitivity ofdimensions on the wafer patterns with respect to dimensional variationof mask patterns, the sensitivity calculations being illustrated as step514.

The shot list 516 may be read by a proximity effect correction (PEC)refinement step 518, in which shot dosages are adjusted to account forbackscatter, fogging, and loading effects, creating a final shot listwith adjusted dosages 520. The final shot list with adjusted dosages 520is used to generate a surface in a mask writing step 522, which uses acharged particle beam writer such as an electron beam writer system.Depending on the type of charged particle beam writer being used, PECrefinement 518 may be performed by the charged particle beam writer.Mask writing step 522 may comprise a single exposure pass or multipleexposure passes. The electron beam writer system projects a beam ofelectrons through a stencil onto a surface to form a mask image 524comprising patterns on the surface. The completed surface, such as areticle, may then be used in an optical lithography machine, which isshown in a step 526. Finally, in a step 528, an image on a substratesuch as a silicon wafer is produced.

FIG. 6 is an exemplary conceptual flow diagram 600 for forming a patternon a substrate such as a wafer, starting from a previously-generatedshot list, and performing mask process correction. Flow 600 begins withoriginal shot list 604, which may comprise shots for one exposure passor for multiple exposures passes. In step 606 sensitivity of the waferpattern dimensions to variation in the resist exposure used in maskwriting is calculated. In one embodiment the method illustrated inconceptual flow diagram 400 (FIG. 4) may be used for step 606. Inanother embodiment, step 606 also uses as input information 602 aboutthe areas in the design where a higher level of dimensional uniformityis required on the wafer. If step 606 determines that the mask patternthat would be generated by original shot list 604 would produce patternareas of excessive sensitivity on the wafer, the shots are modified instep 610. In some embodiments the shot modification 610 may compriseincreasing dosage of shots which form the mask patterns which form areasof high sensitivity in the wafer pattern. In other embodiments shotmodification 610 may comprise increasing dosage only near the perimeterof the mask patterns which form areas of high sensitivity in the waferpattern. Shot modification 610 may comprise generating shots whichoverlap within an exposure pass. Shot modification 610 creates amodified shot list 612. In some embodiments, the wafer sensitivity usingmodified shot list 612 may be re-calculated in a second pass of step606. If re-calculation is performed and sensitive parts of the waferpattern are still found, additional shot modification may be performedin a second pass of step 610, creating a further-modified shot list 612.

In a proximity effect correction (PEC) refinement step 614, shot dosagesmay be adjusted to account for backscatter, loading and fogging effects,creating a final shot list 616. The final shot list 616 is used togenerate a surface in a mask writing step 618, which uses a chargedparticle beam writer such as an electron beam writer system. Dependingon the type of charged particle beam writer being used, PEC refinement614 may be performed by the charged particle beam writer. Mask writingstep 618 may comprise a single exposure pass or multiple exposurepasses. The electron beam writer system projects a beam of electronsthrough a stencil onto a surface to form a mask image 620 comprisingpatterns on the surface. After further processing steps, the completedsurface may then be used in an optical lithography machine, which isshown in a step 622, to produce an image on a substrate such as asilicon wafer 624.

In other embodiments, flow 600 may be modified so that in step 606,wafer pattern dimensional sensitivity to changes in mask patterndimensions is calculated. As above, step 610 may comprise increasingdosage near the perimeter of the mask patterns which form areas of highsensitivity in the wafer pattern, and may also comprise generating shotswhich overlap within an exposure pass.

There are other factors besides resist exposure variation that mayundesirably affect CD of mask patterns and of the subsequently exposedwafer patterns. For example, variation in shot placement and variationin VSB shot size are other factors that can negatively affect CD. MonteCarlo simulations can be done in which random positional, size anddosage errors are introduced to individual shots, so as to determine theeffects of the combined variations. These simulations indicate that theembodiments set forth herein to generate and modify shot lists toproduce wafer patterns which have a dimensional sensitivity below apre-determined limit do, in fact, produce similar dimensionalsensitivities even when variations of shot position and shot size areincluded in the simulations.

The calculations described or referred to in this disclosure may beaccomplished in various ways. Generally, calculations may beaccomplished by in-process, pre-process or post-process methods.In-process calculation involves performing a calculation at the timewhen its results are needed. Pre-process calculation involvespre-calculating and then storing results for later retrieval during asubsequent processing step, and may improve processing performance,particularly for calculations that may be repeated many times.Calculations may also be deferred from a processing step and then donein a later post-processing step. An example of pre-process calculationis pre-calculating a shot configuration that will produce a minimum maskor wafer CD variation for a given situation, and storing informationabout this shot configuration in a table. Another example of pre-processcalculation is a shot group, which is a pre-calculation of dosagepattern information for one or more shots associated with a given inputpattern or set of input pattern characteristics. The shot group and theassociated input pattern may be saved in a library of pre-calculatedshot groups, so that the set of shots comprising the shot group can bequickly generated for additional instances of the input pattern, withoutpattern re-calculation. In some embodiments, the pre-calculation maycomprise simulation of the dosage pattern that the shot group willproduce on a reticle. In other embodiments, the shot group may bedetermined without simulation, such as by using correct-by-constructiontechniques. In other embodiments the pre-calculation may comprisecalculation of wafer or mask dimensions to variation in resist exposure.In some embodiments, the pre-calculated shot groups may be stored in theshot group library in the form of a list of shots. In other embodiments,the pre-calculated shot groups may be stored in the form of computercode that can generate shots for a specific type or types of inputpatterns. In yet other embodiments, a plurality of pre-calculated shotgroups may be stored in the form of a table, where entries in the tablecorrespond to various input patterns or input pattern characteristicssuch as pattern width, and where each table entry provides either a listof shots in the shot group, or information for how to generate theappropriate set of shots. Additionally, different shot groups may bestored in different forms in the shot group library. In someembodiments, the dosage pattern which a given shot group can produce mayalso be stored in the shot group library. In one embodiment, the dosagepattern may be stored as a two-dimensional (X and Y) dosage map called aglyph.

The fracturing, mask data preparation, shot list modification andpattern formation flows described in this disclosure may be implementedusing general-purpose computers with appropriate computer software ascomputation devices. Due to the large amount of calculations required,multiple computers or processor cores may also be used in parallel. Inone embodiment, the computations may be subdivided into a plurality of2-dimensional geometric regions for one or more computation-intensivesteps in the flow, to support parallel processing. In anotherembodiment, a special-purpose hardware device, either used singly or inmultiples, may be used to perform the computations of one or more stepswith greater speed than using general-purpose computers or processorcores. In one embodiment, the special-purpose hardware device may be agraphics processing unit (GPU). In another embodiment, the optimizationand simulation processes described in this disclosure may includeiterative processes of revising and recalculating possible solutions, soas to minimize either the total number of shots, or the total chargedparticle beam writing time, or some other parameter. In yet anotherembodiment, an initial set of shots may be determined in acorrect-by-construction method, so that no shot modifications arerequired.

FIG. 8 illustrates an example of a computing hardware device 800 thatmay be used to perform the calculations described in this disclosure.Computing hardware device 800 comprises a central processing unit (CPU)802, with attached main memory 804. The CPU may comprise, for example,eight processing cores, thereby enhancing performance of any parts ofthe computer software that are multi-threaded. The size of main memory804 may be, for example, 64 G-bytes. The CPU 802 is connected to aPeripheral Component Interconnect Express (PCIe) bus 820. A graphicsprocessing unit (GPU) 814 is also connected to the PCIe bus. Incomputing hardware device 800 the GPU 814 may not, despite its name, beconnected to a graphics output device. Rather, GPU 814 may be usedpurely as a high-speed parallel computation engine. The computingsoftware may, with the use of appropriate GPU interface software, obtainsignificantly-higher performance by using the GPU for a portion of thecalculations, compared to using CPU 802 for all the calculations. TheCPU 802 communicates with the GPU 814 via PCIe bus 820. In otherembodiments (not illustrated) GPU 814 may be integrated with CPU 802,rather than being connected to the PCIe bus. Disk controller 808 is alsoattached to the PCIe bus, with, for example, two disks 810 connected todisk controller 808. Finally, a local area network (LAN) controller 812is attached to the PCIe bus, and provides Gigabit Ethernet (GbE)connectivity to other computers. In some embodiments, the computersoftware and/or the design data are stored on disks 810. In otherembodiments, either the computer programs or the design data or both thecomputer programs and the design data may be accessed from othercomputers or file serving hardware using the GbE Ethernet.

While the specification has been described in detail with respect tospecific embodiments, it will be appreciated that those skilled in theart, upon attaining an understanding of the foregoing, may readilyconceive of alterations to, variations of, and equivalents to theseembodiments. These and other modifications and variations to the presentmethods for fracturing, mask data preparation, shot list modificationand optical proximity correction may be practiced by those of ordinaryskill in the art, without departing from the spirit and scope of thepresent subject matter, which is more particularly set forth in theappended claims. Furthermore, those of ordinary skill in the art willappreciate that the foregoing description is by way of example only, andis not intended to be limiting. Steps can be added to, taken from ormodified from the steps in this specification without deviating from thescope of the invention. In general, any flowcharts presented are onlyintended to indicate one possible sequence of basic operations toachieve a function, and many variations are possible. Thus, it isintended that the present subject matter covers such modifications andvariations as come within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method for mask process correction (MPC) orforming a reticle pattern on a resist-coated reticle, the methodcomprising: inputting pattern exposure information for a chargedparticle beam writer, wherein the pattern exposure information iscapable of forming the reticle pattern on the reticle to be used to forma wafer pattern on a substrate using optical lithography; inputtingresist exposure information for the reticle; calculating a calculatedpattern on the reticle using the pattern exposure information and theresist exposure information; calculating a sensitivity of the waferpattern to changes in the calculated reticle pattern, wherein thecalculating is performed using a computing hardware device; andmodifying the pattern exposure information to increase the edge slope ofthe calculated pattern on the reticle in areas where the wafer patternsensitivity is higher than a predetermined threshold.
 2. The method ofclaim 1 wherein the charged particle beam writer is a shaped beamcharged particle beam writer, and wherein the charged particle beamexposure information comprises a plurality of shots for the shaped beamcharged particle beam writer.
 3. The method of claim 2 wherein themodifying comprises generating a shot which overlaps another shot in theplurality of shots.
 4. The method of claim 1 wherein the chargedparticle beam writer is a multi-beam charged particle beam system, andwherein the charged particle beam exposure information comprisesexposure instructions for the multi-beam charged particle beam exposuresystem.
 5. The method of claim 1, further comprising forming the reticlepattern on the reticle with the modified pattern exposure informationand the charged particle beam writer.
 6. The method of claim 1 whereinthe modifying comprises increasing dosage delivered to the reticle nearthe perimeter of the reticle pattern.
 7. The method of claim 1 whereinthe calculating the calculated pattern on the reticle comprises chargedparticle beam simulation.
 8. A system for mask process correction, thesystem comprising: a device configured to input pattern exposureinformation for a charged particle beam writer, wherein the patternexposure information is capable of forming a reticle pattern on areticle to be used to form a wafer pattern on a substrate using opticallithography; a device configured to input resist exposure information; adevice configured to calculate a calculated reticle pattern from thepattern exposure information and the resist exposure information; adevice configured to calculate a sensitivity of the wafer pattern tochanges in the calculated reticle pattern; and a device configured tomodify the pattern exposure information in areas of the pattern wherethe calculated sensitivity is higher than a predetermined threshold. 9.The system of claim 8 wherein the device configured to modify increasesdosage delivered to the reticle near the perimeter of the reticlepattern.
 10. The system of claim 8 wherein the device configured tocalculate a calculated reticle pattern performs charged particle beamsimulation.
 11. The system of claim 8 wherein the charged particle beamwriter is a shaped beam charged particle beam writer, and wherein thecharged particle beam exposure information comprises a plurality ofshots for the shaped beam charged particle beam writer.
 12. The systemof claim 11 wherein the device configured to modify generates a shotwhich overlaps another shot in the plurality of shots.